Many autoimmune diseases are much more prevalent in women compared to men, and include multiple sclerosis, arthritis and systemic lupus erythematosus. Although the reason for this difference is not currently known, the sex hormones are likely to play a significant role. A population of naturally occurring regulatory T cells, CD4+CD25+ regulatory T cells, has recently been described that is responsible for controlling autoimmune disease in mice. Elimination of this population in mice results in severe multi-organ autoimmune diseases. A homologous population has been found in humans that appears to express similar functional properties, at least, in in vitro assays. Our preliminary results indicate that both quantitative and qualitative differences exist in the CD4+CD25+ regulatory T cell population in adult, but not pre-pubertal, female versus male mice, i.e., CD4+CD25+ regulatory T cell numbers and function are decreased in adult female mice. The central hypothesis of this grant proposal is that sex steroids mediate the gender differences in CD4+ CD25+ regulatory T cells, and through this mechanism may influence the differential expression of autoimmune disease in females versus males. The following specific aims are designed to test this hypothesis: I) Characterization of CD4+CD25+ regulatory T cells in female and male mice; 2) Analysis of sex steroid influences on CD4+CD25+ regulatory T cells; and 3) Identification of the anatomical compartment(s) responsive to sex steroids that influences CD4+CD25+ regulatory T cells. The results of these studies will provide important information that can lead to the development of novel therapies for the prevention and treatment of autoimmune disease.